The present invention relates generally to volumetric type rotary machines and more particularly to improvements in volumetric rotary machines of the kind having a pair of screw rotors each of which has a cross sectional shape of a toothed wheel with crests and valleys in planes perpendicular to the rotational axis and is twisted in its rotational axis direction, the two rotors rotating in mutually meshed state.
Heretofore, among rotary machines such as flowmeters, pumps, motors, fans, blowers, and compressors, there have been those having Roots type rotors, elliptical-gear type rotors, and the like. In a Roots type rotary machines, the rotors rotate smoothly even when the fluid flowing through the rotary machine lacks lubricity. Furthermore, in order to cause the rotors to rotate at equal rotational speed, the rotors are prevented from directly contacting each other, and, moreover, mutually meshing timing gears are respectively provided at the ends of the rotor shafts of the rotors. A problem accompanying this Roots type rotary machine is that, because of the necessity of providing timing gears therein in this manner, the machine becomes complicated and cannot be produced at low cost.
The above mentioned elliptical-gear type rotary machine does not require timing gears, but the fabrication of the rotors is very troublesome and costly since it is necessary to cut gear teeth on the elliptical rotors. Furthermore, the rotational speeds of the rotors are not equal, and the fluids handled by this machine are limited to those of lubricative nature.
Still another problem common to both of the above mentioned rotary machines is that a pulsative flow develops in the fluid which has passed through the machine as the rotors rotate as a result of the periodic release, on the outlet side, of the fluid which has been confined on the inlet side by the rotors between the rotors and the casing.
An example of a rotary machine having rotors of a shape differing from that of the above described rotors is the screw compressor having a pair of screw or helical rotors as proposed in the U.S. Pat. No. 3,986,801. This screw compressor has two screw rotors each of which, as viewed in cross section in a plane perpendicular to the rotational axis, has crests and valleys comprising circular arcs around its periphery at equal intervals and is helically twisted in the direction of the rotational axis. The crests and valleys of these screw rotors are directly meshed with each other, and these rotors thus rotate in the delivery direction when one of the rotors is positively driven in rotation from the outside. As a result, the fluid being compressed is sent in the rotational axis direction along the helical grooves of the screw rotors and is compressed as the meshing action of the screw rotors takes place.
Since the two screw rotors rotate in directly meshed state in this screw compressor, timing gears are unnecessary, and there is no necessity of forming small gears on the rotors. Accordingly, the compressor can be easily produced at low cost. Furthermore, since the fluid from the inlet side is sent in a relatively continuous manner to the delivery outlet side as the rotors rotate, whatever pulsative flow that develops in the fluid is very small, and the level of the noise generated is also very low.
Each rotor of this screw compressor, however, has a toothed profile in cross section wherein the peripheral figure of the tooth crests and tooth valleys comprise only circular arcs of the same radius. For this reason, and as described also hereinafter, a gap is formed as a natural result at the meshing parts of the pair of screw rotors. Consequently, the inlet side and the outlet side become directly communicative through this gap, whereby the performance of the machine as a compressor is not very high.
It appears that applications of the principle and mechanism of this screw compressor to other rotary machines such as a flowmeter, for example, were not conceived, but if they were applied to a flowmeter, the accuracy of flow measurement of such a flowmeter would be impaired by the above mentioned communicative gap.
The above described screw rotors are accompanied by other difficulties such as unequal rotational speeds of the rotors in the vicinity of the pitch point giving rise to slipping, rapid wear, and poor durability and difficulty in fabrication.